23 May 2013

Baits failing as cockroaches adapt to dislike sugar

by Will Parker

Sugar is losing its attraction for roaches and making baits less effective, say scientists who have been investigating the genetic adaptations that are causing cockroaches to increasingly reject glucose.

In a study in the journal Science, North Carolina State University entomologists say the aversion has a genetic basis and it eventually spreads to offspring, resulting in growing numbers of glucose-averse cockroaches.

"Most times, genetic changes, or mutations, cause the loss of function," said Coby Schal, one of the authors of the paper. "In this case, the mutation resulted in the gain of a new function - triggering bitter receptors when glucose is introduced. This gives the cockroach a new behavior which is incredibly adaptive. These roaches just got ahead of us in the arms race."

In normal cockroaches, glucose elicits activity in sugar gustatory receptor neurons, which react when exposed to sugars like glucose and fructose - components of corn syrup, a common roach-bait ingredient. Generally, roaches have a sweet tooth for these sugars.

In the study, the researchers conducted tests on the roach tongue, the paired mouth appendages called paraglossae. The tests showed that glucose stimulates both sugar and bitter receptor neurons, confirming behavioral tests that showed roaches quickly fleeing from glucose when presented with it.

"We don't know if glucose actually tastes bitter to glucose-averse roaches, but we do know that glucose triggers the bitter receptor neurons that would be triggered by caffeine or other bitter compounds," explains Schal. "That causes the glucose-averse roach to close its mouth and run away from glucose in tests."

There appears to be a cost, however, to cockroaches with glucose aversion. In the absence of glucose-toxicant mixtures, glucose-averse cockroaches grow more slowly than normal roaches in laboratory settings where there are no nutritional stresses. "Now we want to understand how this trait persists in nature, where the food supply is probably limited," co-researcher Jules Silverman said. "Cockroaches have to adapt to a varied and unreliable food supply, and glucose-aversion places an additional restriction on obtaining adequate nutrition."